Method of measuring the quality of an optical slit



NOY. 21x9 VOLUME DDDDDEIUDEIUDDEIUUDU H. c. WOH LRAB 2.1809 2 METHOD OF MEASURING THE QUALITY OF AN OPTICAL SLIT Filed April 12, 1938 2 Sheets-Sheet 1 aoau souNo TRACK uuussa DISPLACEMENT (0.: MM)

INVENTOR H. C. WOHLRAB A TTORNEV 21.1939. H. c. WOHLRAB 1 2.180.972

METHOD OF MEASURING THE QUALITY OF AN OPTICAL SLIT Filed April 12, 1958 2 Sheets-sheaf. 2

VOLUME o i 1'1 m 1y \'1 1': 11'11 121 n XIIXIIIIIYXYSOUNwDMBEZACK -aao' DECLINATION +33o' A INVENTOR H. C. WOHLRAB BY A TTORNEV -several sound tracks are used for these measure is displaced by a con a a e With respect to 10 .20 displacement of the sound track by a certain travel of afilm the power delivered is ,20

Patented Nov. 21, 1939 Zifitfifig UNITED s'rArss PATENT orrrca METHOD OF MEASURING THE QUALITY OF AN OPTICAL SLIT Hans O. Wohlrab, Berlin-Wilmersdorf, Germany,

assignor to Klangfilm Gesellschaft mit beschran'kter Haftung, Berlin, Germany, a German company Application April 12, 1938, Serial No. 201,480 In Germany April 29, 1937 3 Claims. (01. 179-1003) The present invention relates to a method of position of the slit with respect to the perforameasuring the quality of scanning beams in tion can be determined. sound film apparatus. In particular the invention In order to determine the obliquity of the slit describes sound films by means of which the a second film may be used, this having several 5 length of the beam, theposition of the beam with sound tracks arranged one behind the other, for

respect tothe perforation, the distribution of example in variable densityrecording. The varillumination throughout the beam and the obious sound tracks have an angul posififin With liquity of the beam can be measured. respect to the direction of film movement and According to the invention, sound films with are preferably wider than the slit. Each of them ments. These sound tracks have a certain posithe next one; forexample, the arrangement may tion with respect to the longitudinal direction be such that one track is recorded at right angles of the film,.as will be described below, Onesound to thedirection of .film .movement while the prefilm is provided with a record in interrupted ceding one deviates by a certain angle in one rectangular form, but, of course, other forms of direction with regard to thistrack, while the one 5 recording may be used instead. Preferablythis following this sound track recorded at ri ht rectangular record has a uniform width over angles tozthe film movement, deviates by the same the entire track and is so arranged on the film angle in the other direction.

band in the direction of the film that a lateral Now it is a well-known fact that during the amount alwaystakes place after a certain extent when the .record striation and the reproducing of .film travel. The laterally displaced sound beam are parallel. Thus, if a film having series records are of equal constant frequency and of records as previously described is passed amplitude so that all the elemental areas of light through a reproducing apparatus, the power deat the slit will be modulated equally. The total livered will vary with the angularity of the por- 25' width of this record is wider than the reproductionof the record being rep ding slit. Thearrangement of displaced sound Examples of the present invention wil now be tracks may follow in direct succession. Any other explained with the aid of the figures. soundrecord, for example a word text, may how- Fig. 1 shows a normal sound film;

ever also be recorded between two tracks in the Fig. 2 a sound film strip to an enlarged scale 30 longitudinal direction. for measuring the position and the illumination Now this film is passed through the reproducof the slit; ing apparatus. Associated with the photocell for Fig. 3 is the curve recorded with the film acinstance is a rectifier with a measuring instrument cording to Fig. '2;

upon which thepower delivered from the photoi Fig. 4 shows afilm for measuring the obliquity 35 cell may be read. If this sound film is run of the slit; through the reproducing apparatus, the first Fig. 5 is the curve taken withthis'film. sound track will still not produce any impulses on In these drawings the same reference numthe photocell, if it lies outside the slit. Perhaps erals are used for the same parts.

40 the second sound track already lies within the slit, Fig. 1 shows the usual sound film with the per- 40 so that light falls on the photocell. The power deforation holes A and the sound track B. The livered may now be read at the measuring insurface Cserves for receiving the picture. The strument. As the entire film runs successively sound track B has a certain width a and a certain through the reproducing device, the power demargin distance 12-. Further, the distance 0 of livered by each individual sound track can be the sound track from the edge of the perforations 45 measured until the record again lies outside the is indicated. slit. As the width of slit and therefore the slit Now Fig. 2 shows a sound film to a large illumination is not perfectly uniform, the descale such as it is intended to use according to livered power will fluctuate so that by means of the invention. The reproducing slit D is inthe various measuring points the exact illuminadicated with dotted lines. This sound film may 50 tion of the slit can be determined. The first have, for example, a record of 800 cycles in recrecord is so arranged that a certain distance is tangular form. The various sound tracks of maintained from the perforation. The width of which 31 are provided, are about 1.5. meters long the various records is preferably equal. C'onseand are displaced by an amount at, which accordquently both the length of the slit and also the ing to the invention is 0.1 millimeter. Between 55 the various sound tracks in the longitudinal direction there is a text inscription in which the current number of the sound track is given, for example, 1, 2, 3, etc. The total width of the sound track thus amounts to 3l 0.l=3.1 millimeters. The width will be so chosen that it is greater than that of the reproducing slit. In the film, in accordance with the invention, as shown in Fig. 2, the longitudinal strip of film adjacent one row of film perforations in which the laterally displaced records are to be located is made opaque with the exception of the area occupied by each of the laterally displaced records, so that no light will reach the photoelectric cell except that admitted by sound records.

The measurement is carried out as follows: The film is placed in the reproducing apparatus and played off. The sound track I still lies outside the slit so that no light still falls upon the photocell and consequently still no performance is indicated at the associated measuring instrument. Moreover, the sound tracks 2-4 will also lie outside the slit. Only at the sound track 5 does light fall through the slit. The same applies also to the sound tracks 6-25. The delivered power which is brought about by the various sound tracks can be plotted on a graph sheet as indicated in Fig. 3. This graph must have, for example, a linear scale in both axial directions. Theoretically, with an ideal slit the various sound track measurement points should give a line parallel with the X-axis (indicated dotted) if the slit illuminations were uniform. This, however, is not usually the case; thus, some form of curve ordinarily results, Fig. 3 illustrating a not uncommon variety. From this curve it is now possible to ascertain to what extent the slit illumination is uniform. Furthermore, it is also possible to ascertain that the slit begins at the sound track 5 and ends at the track 25, that is, it has a width of 2.0 millimeters. As the distance of the first record from the edge or from the perforations is fixed, it is possible to read from the record what distance the slit is from the perforation.

Fig. 4 shows the sound film by means of which the slit obliquity can be measured. According to the invention, it has 15 sound tracks which are arranged in the longitudinal direction of the film one behind the other. Each sound track is 1.5 meters long and may have a 5,000 cycle variable density record. Between the various sound tracks there are sound registrations which contain the numbers and the position of the sound tracks. In the drawings the record VIII forms an angle of 90 degrees with the longitudinal direction of the film. Towards the smaller numbers of the sound track, this angle will diminish by a certain angle, for example, 30 minutes, and to the larger sound track numbers it will increase by the same amount. In its present form the record will have at least the width of the slit. Now, if such a film is allowed to run through the reproducing device and if the power delivered by the photocell is measured, the curve obtained by plotting the values will be, for example, as shown in Fig. 5. From this curve it will be apparent that the maximum of the power lies at the point IX. If the power at the points VIII and X were equal it would indicate that the slit is parallel with the record striation of record IX and, therefore, have an inclination with respect to normal amounting to +30. However, the curve shows that the power at point VIII is greater than the power at point X, indicating that the slit has an inclination between normal, as represented by record VIII, and +30 with respect to normal as represented by record IX.

Thus, it is clear that by means of the invention a simple and accurate measurement of the quality of a reproducing slit may be made.

What is claimed is:

l. The method of determining the uniformity of illumination of a fixed slit in a film sound recording system which comprises illuminating said slit, successively modulating the light at a plurality of transverse elemental areas of the slit by a sound record of constant frequency and amplitude, translating the record into electrical impulses, and measuring the power of said impulses.

2. In a sound film apparatus having an optical system including an illuminated member having a fixed slit therein, a film provided with a plurality of successive sound records equal in amplitude and of the same frequency each of said records having a width which is a fraction of the length of the slit in said member, the successive records being displaced laterally from the marginal film edge by predetermined amounts, means for moving said film past the fixed slit in said member, and means for converting said sound records into electrical impulses.

3. A test record including a plurality of successive sound records equal in amplitude and of the same frequency, each of said records having a lateral dimension smaller than the lateral dimension of the light beam in optical systems to be tested thereby, the successive records being laterally displaced progressively increasing distances from the marginal film edge.

HANS C. WOHLRAB. 

